The oligo- or polymerization of isocyanates, especially to form higher molecular weight oligomer mixtures having uretdione (“dimer”), isocyanurate (“trimer”) and/or iminooxadiazinedione structures (“asymmetric trimer”) in the molecular skeleton, has long been known. As can be seen above, the oligo- and polymerization of isocyanates are based in principle on the same chemical reactions. The reaction of a relatively small number of isocyanates with one another is referred to as oligomerization. The reaction of a relatively large number of isocyanates is referred to as polymerization. In the context of the present invention, the oligomerization or polymerization of isocyanates described above is referred to collectively as isocyanate modification or modification of isocyanates.
The modified polyisocyanates comprising free NCO groups, which optionally may also have been temporarily deactivated with blocking agents, are exceptionally high-quality starting materials for the preparation of a multiplicity of polyurethane plastics and coating compositions.
A series of industrial methods for isocyanate modification have been established in which the isocyanate to be modified, usually a diisocyanate, is generally reacted by addition of catalysts and these are then rendered inactive (deactivated) by suitable measures, when the desired degree of conversion of the isocyanate to be modified has been reached, and the polyisocyanate obtained is generally separated from the unreacted monomer. A summary of these methods from the prior art can be found in H. J. Laas et al., J. Prakt. Chem. 1994, 336, 185 ff.
Compounds with an ionic structure have proven to be effective as modification catalysts since they may be used in very low amounts, relative to the monomer to be reacted, and lead extremely rapidly to the desired result.
The option of also using tetraorganylammonium or -phosphonium as cation to the anion which is catalytically active toward isocyanates, such as hydroxide, alkanoate, alkoxylate, etc., is common knowledge, although generally not explicitly emphasized as being particularly preferred; cf.: H. J. Laas et al., J. Prakt. Chem. 1994, 336, 185 ff.
Additionally known is the use of fluorides and hydrogenpolyfluorides, the latter being stable adducts of HF onto compounds containing fluoride ions, optionally also in the form of their ammonium or phosphonium salts, for the isocyanate modification, from documents including EP 962 455 A1, EP 962 454 A1, EP 896 009 A1, EP 798 299 A1, EP 447 074 A1, EP 379 914 A1, EP 339 396 A1, EP 315 692 A1, EP 295 926 A1 and EP 235 388 A1.
However, the tetraorganylammonium and -phosphonium (hydrogenpoly)fluorides of the prior art, in the performance of the modification reaction, have the disadvantage that, when they are used, the reaction can sometimes be maintained only with continuous metered addition of catalyst, meaning that the breakdown of the catalyst in the isocyanate medium proceeds unacceptably quickly for technical purposes compared to the modification reaction.
An additional factor is that, when tetraorganylammonium (hydrogen)polyfluorides are used, an atypical reaction profile is sometimes observed, which leads to products having a much lower iminooxadiazinedione group content than in the case of a regular heat production rate profile (cf. EP 962 455 A1). According to the teaching of EP 962 455 A1, this disadvantage was eliminated by the use of phosphonium salts, but the latter—especially at relatively high reaction temperatures—have the unacceptably high tendency to decomposition mentioned further up, and the decomposition products can have an adverse effect on process and product stability.
EP 2 415 795 A1 claims, inter alia, very stable tetraorganylphosphonium (hydrogenpoly)fluorides that do not have these disadvantages, but they are not commercially available and are preparable only with difficulty.